Vitamin D Absorption Issues

If your serum 25(OH)D stays low despite reasonable supplementation, absorption is the usual explanation. Vitamin D is fat-soluble; anything that reduces fat absorption, accelerates catabolism, or shifts the D₃-to-25(OH)D conversion can produce apparent supplementation resistance.

Gastrointestinal causes

• Celiac disease — damaged small-intestinal villi reduce fat and fat-soluble vitamin absorption. Vitamin D deficiency is present in 25–65% of untreated celiac patients and resolves gradually on a gluten-free diet.
• Crohn's disease — ileal inflammation reduces bile acid recycling and impairs fat-soluble vitamin uptake. Higher supplemental doses (3,000–5,000 IU/day) and periodic re-testing are standard.
• Ulcerative colitis — less absorption impact than Crohn's, but chronic inflammation and glucocorticoid use compound the issue.
• Cystic fibrosis — pancreatic insufficiency reduces lipase secretion, which is required for micelle formation. Fat-soluble vitamin supplementation (A, D, E, K) is routine.
• Short bowel syndrome — reduced absorptive surface. Often requires prescription 50,000 IU weekly.
• Cholestatic liver disease — reduced bile salt secretion impairs micelle formation.

Post-bariatric surgery

Roux-en-Y gastric bypass (RYGB) and biliopancreatic diversion (BPD/DS) bypass the duodenum and proximal jejunum, where most vitamin D absorption occurs. Post-op patients typically require 2,000–5,000 IU/day cholecalciferol indefinitely, plus calcium citrate 1,200–1,500 mg/day. Sleeve gastrectomy has a milder effect but still warrants monitoring.

Medications

• Glucocorticoids (prednisone, hydrocortisone) — induce CYP24A1, accelerating 25(OH)D catabolism. Patients on chronic ≥ 5 mg prednisone typically need 2,000–4,000 IU/day.
• Anticonvulsants (phenytoin, carbamazepine, phenobarbital) — same enzyme induction; consider 2× standard dose.
• Cholestyramine and colestipol — bile acid sequestrants block fat-soluble vitamin absorption. Separate doses by 4+ hours.
• Orlistat (Xenical, Alli) — lipase inhibitor; reduces vitamin D absorption. Take supplements at least 2 hours apart from orlistat doses.
• Antiretrovirals — some HIV medications increase 25(OH)D catabolism; monitor annually.

Genetic factors

Polymorphisms in the vitamin D-binding protein gene (GC / DBP), CYP2R1 (25-hydroxylase), and CYP24A1 (24-hydroxylase) each explain 1–4% of individual variation in serum 25(OH)D. Rare CYP24A1 loss-of-function mutations cause idiopathic infantile hypercalcaemia and require careful, low-dose supplementation. Standard supplementation is safe for the general population.

Troubleshooting checklist

1. Are you taking the supplement with a fat-containing meal? If not, absorption drops 30–50%.
2. Is your dose adequate for your body weight? Obese adults typically need 2× lean-adult dose.
3. Is the product actually cholecalciferol (D₃)? D₂ raises 25(OH)D less effectively.
4. Are you on any of the medications above?
5. Have you been screened for celiac (tTG-IgA), Crohn's, or other GI absorption disorders?
6. Have you tried a liquid oil-based formulation instead of a tablet? Oil vehicles improve absorption in impaired-fat-uptake patients.
7. If all else fails: prescription 50,000 IU weekly cholecalciferol, or investigate 1,25(OH)₂D directly with an endocrinologist.